the service company that i work for makes all employees sign a form stating that we will not compete with the company in any way. while working for the company we cannot work for another company doing the same job at the same time. we were told verbally that we would not need to have another job because they guaranteed 40 hours a week. but know its very slow for our company and they're sending employees home with only a couple of hours worked. they are telling employees that if they want 40 hours, they would have to use their personal or vacation time to make up for the missing hours. what kind of obligation does the company have to keep employees on the clock?

The short answer to your question is none (without a bona fide contract to the contrary).

If you are scheduled for X hours, though, and you report for work as required, you must be paid at least 1/2 those hours with a minimum of 2 hours and a maximum of 4 hours.
http://www.dir.ca.gov/dlse/FAQ_ReportingTimePay.htm

However, I have heard that noncompetes are virtually unenforceable in California, although I don't know if that applies to current as well as for terminated employees, so hold on for further information about that issue.